Widget design system: Material UI

[smhigley]

Related to #252

Look into creating a set of material UI widgets for Dojo, and explore how easily existing Dojo widgets can be modified for material UI. Broad questions that need to be addressed include:

• How flexible is the current approach to widget theming?
• Is it feasible to use themed extraClasses as an approach for adjusting minor styles?
• Even if material widgets need to be rewritten, how difficult is it to extend existing widgets to provide reasonable extra functionality, using material as a test case?
• Are the current meta helpers (animation, focus, etc) flexible and easy to use (again using material as a test case)?
• If we adopt atomic design principles, do we include components higher than molecules?
• What new widgets should we add?

[agubler]

This has been investigated and will be something that is undertaken and tracked in the @dojo/widgets repository over the coming releases.